Fourier transform infrared (FTIR) spectrometers are utilized to perform accurate and efficient identification of the chemical composition of a sample. Such spectrometers typically incorporate an interferometer such as a Michelson interferometer that has a beamsplitter and a moving mirror. The interferometer modulates the beam from a source to provide an output beam in which the intensity of the radiation at various wavelengths is varied. The light may be in the near ultraviolet (UV), visible (Vis), near-infrared (NIR), mid-infrared (MIR), and/or far-infrared (FIR) wavelength ranges, and thus, is not limited to the infrared spectral region. The output beam is focused and passed through or reflected from a sample, after which the beam is collected and focused onto a detector. The detector provides a time varying output signal which contains information concerning the wavelengths of absorbance or reflectance of the sample. For example, the intensity of the output light at the one or more wavelengths is compared to the intensity of the input light at the one or more wavelengths to determine characteristics of the sample, such as the absorbance, the transmittance, the fluorescence, the reflectance, etc. Fourier analysis is performed on the output signal data to yield the measured characteristics that provide information about the identity of the components within the sample, their relative concentrations, and possibly other features of the sample.
Conventional FTIR spectrometers include a sample chamber in which a sample is held in a position to be exposed to the light from the interferometer. The sample may take various physical states, i.e., a liquid, a solid, or a gas, and solid samples may have various physical characteristics. For example, a solid material to be analyzed may be in the form of a block or sheet of material (e.g., polymer plastics), in the form of powders or granulates, or in specific formed shapes (e.g., pharmaceutical tablets, pills and capsules).
Multifunctional FTIR spectrometers perform transmission or reflection measurements, or both, on a variety of samples, including liquids and powders as well as shaped solid samples such as pharmaceutical pills and tablets. The various samples can be tested utilizing the same spectrometer system without modification of the spectrometer and without the addition or rearrangement of sample compartments and sample holders. The spectrometer includes a plurality of sample holders configured within a transmission or reflection measurement system.
Attenuated total reflectance (ATR) is a sampling technique used in conjunction with infrared spectroscopy that enables samples to be examined directly in the solid, liquid, or gas state without further preparation. ATR uses a property of total internal reflection resulting in an evanescent wave. Light is passed through a crystal in such a way that it reflects at least once off the internal surface in contact with the sample. This reflection forms the evanescent wave which extends into the sample. The penetration depth into the sample is determined by the wavelength of light, the angle of incidence, and the indices of refraction for the crystal and the medium being probed. The number of reflections may be varied by varying the angle of incidence. The beam is collected by a detector as it exits the crystal. Example materials for ATR crystals include germanium, zinc selenide, and diamond.